1. Field of the Invention
This invention relates to the field of mechanical gear systems. More specifically, the invention comprises a controlled damper motor placed between the gearbox output and the load.
2. Description of the Related Art
Many mechanical systems require the use of speed-changing gears. These are conventionally housed in a gearbox, which may include two or more sets of meshing gears. FIG. 1 shows one simple depiction of such a system. Prime mover 24 is intended to provide rotational mechanical energy to load 28. Unfortunately, the speeds at which the two devices operates are incompatible. The prime mover in this example is a steam turbine, which typically operates in excess of 10,000 RPM. The load is a synchronous electric generator, which operates at 360 RPM. The prime mover and the load may obviously not be directly connected.
The solution to this problem is the use of reduction gearbox 26. Reduction gearbox 26 houses several sets of meshing gears. Input shaft 30 is spinning at the speed of the turbine, but output shaft 32 is reduced to the rotational velocity needed for load 28.
While the reduction gearbox solves the speed compatibility issue, it introduces other problems. Those skilled in the art will know that every set of mating gears creates a backlash issue. The term backlash generally means the amount of rotational “slop” which exists between two mating gears. If an input shaft feeds torque into a single set of mating gears and an output shaft transmits that torque from the mating gears, it will be possible to turn the input shaft back and forth through some amount of rotation without turning the output shaft (and vice versa). This amount is referred to as backlash.
When the input and output shafts are loaded (such as by transmitting a fixed torque from a prime mover to a load) backlash is not typically a problem since the gear teeth remain engaged. However, when the load varies (or the system is unloaded or reversed), the gear teeth may become transiently disengaged and reengaged. This can produce resonant problems, as well as excessive gear tooth wear.
The problems associated with backlash have traditionally been addressed through gear design. Some of the prior art solutions include (1) designing specialized gear teeth which are better suited for dynamic loading conditions; (2) reducing tolerances between the interfacing gears; and (3) adding a fixed biasing torque which keeps the teeth engaged in one direction. Gear systems employing these solutions are often expensive to manufacture. The use of such specialized gears may reduce “clanging” and vibration. However, the gears still experience additional stress under dynamic loading and unloading conditions. Furthermore, tight gear interface tolerances often increase mechanical friction and present lubrication problems.
Backlash within a gear system also causes problems with resonance. A gear train possesses one or more critical speeds. When operated around these critical speeds, cyclic torsional vibration tends to increase. Vibration obviously reduces gear and bearing life. However, in many large systems, it is simply unsafe to operate the gear train at a critical speed. The system must then be designed to quickly pass through this speed (both accelerating and decelerating) in order to avoid damage.
An example of a rotating mechanical system is shown in FIG. 2. Motor 1 (34) and motor 2 (36) are connected to rotate in unison. These are collectively an input torque generating device. Motor 1 drive (44) and motor 2 drive (46) control motor 1 and motor 2, respectively. The two motors feed into low speed gear box 38. This component increases the rotational speed before feeding into high speed gearbox 40, which further increases the rotational speed. The output of high speed gearbox 40 is connected to load 28.
Such an arrangement can be used to evaluate the performance of the components involved. It is desirable to be able to control the input torque and the load torque. The motor drives control the input torque while load controller 42 controls the load torque. The reader may wish to consider exemplary specifications for the system of FIG. 2. Motor 1 and motor 2 are each 2.5 MW variable speed motors capable of producing a peak torque of 78,220 ft-lbf@225 RPM. Low speed gearbox 38 has an input range of 0-450 RPM and an output range of 0-3,600 RPM. High speed gearbox 40 has an input range of 0-3,600 RPM and an output range of 0-24,000 RPM. The load in this example spins in the range of 0-24,000 RPM and consumes a maximum of 5 MW.
Highly precise control of the input and output shaft torques can avoid the backlash and resonance problems discussed previously. However, those skilled in the art will realize that such control is often impossible. Returning to the example of FIG. 1, the reader will appreciate that one cannot rapidly adjust the torque produced by a prime mover such as a steam turbine. By the same token, one cannot rapidly adjust the torsional load created by a device such as a large electrical generator. Thus, it is desirable to introduce a new element which can provide rapid torque control to minimize backlash-related problems. The present invention proposes just such a device.